Gwinnett County judges have created a special grand jury to investigate land purchases by county government, including parks and sites for libraries, highways and baseball. D.A. Danny Porter said he has already assembled details of 41 parkland buys since 2005, to be followed by particulars on purchases for libraries, rights of way and other property — “probably” including the $5 million purchase of 12 acres for the minor-league Gwinnett Braves’ stadium.